Photomasks are high precision plates containing microscopic images of electronic circuits. Photomasks are typically made from flat pieces of quartz or glass with a layer of chrome on one side. Etched in the chrome is a portion of an electronic circuit design, often referred to as “geometry”. Photomasks are used in wafer fabrication, mainly to make integrated circuits (“ICs”) and other semiconductor devices. In turn, ICs are used in a variety of different products, including computers, calculators, cars, cameras, stereos, etc. Photomasks are also used to make flat panel displays, thin film heads, PC boards, and other electronic products.
One type of photomask known in the art is an embedded attenuated phase shift mask (“EAPSM”). EAPSMs are used in the production of semiconductor devices, and more particularly, EAPSMs are typically used to print contact layer holes in a semiconductor wafer. As shown in FIG. 1, a typical blank EAPSM 10 is comprised of four layers. The first layer is a layer of quartz or other substantially transparent material 11, commonly referred to as a substrate. The next layer is typically an embedded phase shifting material (“PSM layer”) 12, such as molybdenum silicide (MoSi), tantalum silicon nitride (TaSiN), titanium silicon nitride (TiSiN) or zirconium silicon oxide (ZrSiO) and other known phase materials. The next layer is typically an opaque material 13, such as chromium, which may optionally include an anti-reflective coating such as chromium oxynitride (CrON). The top layer is a photosensitive resist material 14.
The method for processing a conventional EAPSM is now described. The desired pattern of opaque material 13 to be created on the EAPSM 10 may typically be scanned by an electron beam (E-beam) or laser beam in a raster or vector fashion across the blank EAPSM 10. One such example of a raster scan exposure system is described in U.S. Pat. No. 3,900,737 to Collier. As the E-beam or laser beam is scanned across the blank EAPSM 10, the exposure system directs the E-beam or laser beam at addressable locations on the EAPSM. The areas of the photosensitive resist material that are exposed to the E-beam or laser beam become soluble while the unexposed portions remain insoluble.
As shown in FIG. 2, after the exposure system has scanned the desired image onto the photosensitive resist material 14, the soluble photosensitive resist material is removed by means well known in the art, and the unexposed, insoluble photosensitive resist material 14′ remains adhered to the opaque material 13. Thus, the pattern to be formed on the EAPSM 10 is formed by the remaining photosensitive resist material 14′.
The pattern is then transferred from the remaining photoresist material 14′ to the opaque layer 13 and PSM layer 12 via known etch processes, wherein the opaque layer 13 and PSM layer 12 is removed in regions which are not covered by the remaining photoresist 14′. There are a wide variety of etching processes known in the art, including dry etching as well as wet etching, and thus a wide variety of equipment used to perform such etching. After etching is completed, the remaining photoresist material 14′ is stripped or removed and the EAPSM 10 is completed, as shown in FIG. 3. In the completed EAPSM 10, the pattern as previously reflected by the PSM 12′ and opaque materials 13′ is located in regions where the remaining photoresist 14′ remain after the soluble materials were removed in prior steps.
In operation, EAPSMs permit some light from the exposure tools (e.g., semiconductor imaging tools such as wafer steppers) to be transmitted through the opaque layer. In other words, the opaque layer (“the lines”) is partially transmissive. The light that passes through the phase shift layer is engineered to be 180° out of phase with light transmitted through the etched areas (“the space”) in the mask. The phase-shifted electric field amplitude and the non-phase-shifted electric field amplitude destructively interfere with each other. As a result, the net amplitude of light becomes zero in the spaces. The zero amplitude node increases the image contrast and depth of focus. Put another way, the phase shift material enhances areas of transition for bright (e.g., transparent) to dark (e.g., opaque) materials, and thus, allows for more exposure latitude.
To determine if there are any unacceptable defects in a particular photomask, it is necessary to inspect the photomask. A defect is any flaw affecting the geometry of the pattern design. For example, a defect may result when chrome is located on portions of the EAPSM 10 where it should not be (e.g., chrome spots, chrome extensions, or chrome bridging between geometry) or unwanted clear areas (e.g., pin holes, clear extensions, or dear breaks). A defect in an EAPSM can cause a semiconductor to function improperly. To avoid improper function, a semiconductor manufacturer will typically indicate to a photomask manufacturer the size of defects that are unacceptable. All defects of the indicated size (and larger) must be repaired. If such defects cannot be repaired, the mask must be rejected and rewritten.
Typically, automated mask inspection systems, such as those manufactured by KLA-Tencor and ETEC (an Applied Materials company) are used to detect defects. Inspection tools use light transmitted through the EAPSM to find defects in a pattern. In this regard, automated inspection systems direct an illumination beam at the photomask and detect the intensity of the portion of the light beam transmitted through and reflected back from the photomask. The detected light intensity is then compared with expected light intensity, and any deviation is noted as a defect. In this regard, the inspection tool compares the patterned data on the mask to either another part of the mask or to expected pattern data stored in a database. The details of one inspection system can be found in U.S. Pat. No. 5,563,702, assigned to KLA-Tencor. Current inspection equipment is manufactured to operate at a wavelength of 365 nm. Examples of such inspection systems include the KLA-Tencor SLF 77 and AMAT ARIS21-I.
Current inspection tools, however, are often unable to detect defects in conventional EAPSMs. In this regard, phase shift materials used in conventional EAPSMs are deposited on the mask to have specific transmission and phase shift specifications at the exposure tool (e.g., wafer stepper) wavelength, which is currently 248 nm, 193 nm, and 157 nm depending upon the type of exposure tool used. Such current exposure tools require that the EAPSM layer transmit light at a rate of approximately 6-20% relative to the transmission of quartz. Accordingly, current phase shift materials in EAPSMs are tuned to be partially transmissive (e.g., 6-20% relative to quartz) at the exposure tool wavelengths, and thus, meet the optical requirements of exposure tools.
These same phase shift materials, by contrast, are highly transmissive at the greater, inspection tool wavelength (currently 365 nm), thereby making it difficult for inspection tools to detect defects in the EAPSM during inspection. In this regard, current inspection tools require that the phase shift material of the EAPSM transmit light during inspection at a rate of approximately 40-50% or less (depending on the type of inspection tool used) when compared to the transmission of light through the transparent regions (e.g., quartz) of the EAPSM. This is required so that the inspection tools can distinguish between light and dark areas on the EAPSM, thereby making defects apparent to the inspection tool. It should be noted that the particular optical specifications of the inspection tools vary depending upon the type of equipment used. For example, the KLA-Tencor 3XX Series requires a transmission of approximately 40% or less through the PSM layer when compared with quartz. Other inspection tools, such as the KLA-Tencor SLF Series, Lasertec MD2XXX and AMAT ARIS Series, by contrast, require a transmission of 50% or less through the PSM layer when compared with the transmission through quartz. Because, as noted herein, current phase shift materials are highly transmissive at the 365 nm inspection tool wavelength (typically greater than 50% when compared to quartz), the inspection tool cannot distinguish the phase shift material from the from quartz. Phrased another way, the inspection tool is unable to distinguish between light and dark areas in the mask. Thus, it has become increasingly difficult to obtain reliable and accurate inspection results. As a result, the reliability of inspection equipment has become increasingly marginal.
For example, tantalum silicon nitride (TaSiN) has been found to be a good choice of material as the PSM layer 12 for use at the 193 mm exposure tool wavelength. As shown in FIG. 4, TaSiN is substantially opaque at the 193 nm exposure tool wavelength, allowing a transmission of approximately 15% and a 180 degree phase shift. Thus, the TaSiN composite meets the optical specifications necessary for use with conventional exposure tools. TaSiN, however, is highly transmissive at the 365 nm inspection tool wavelength. Referring to FIG. 4, the TaSiN composite material allows approximately 80% transmission of light through the TaSiN PSM layer 12 at the 365 nm inspection tool wavelength, and thus, is substantially transparent during inspection. Because transmission here is outside the acceptable optical range required by current inspection tools (e.g., 40-50% or less), the inspection tool cannot adequately distinguish the TaSiN PSM layer 12 from the quartz layer. As a result, defects in the photomask are not detected, and thus, the semiconductor from which the mask will be made (once the mask is etched or processed) may have imperfections. Accordingly, reliable inspection results cannot be obtained.
After inspection is completed (albeit with unsatisfactory results), a completed photomask is cleaned of contaminants. The cleansing process can also affect the quality of the photomask. Next, a pellicle may be applied to the completed EAPSM to protect its critical pattern region from airborne contamination. Subsequent through pellicle defect inspection may be performed. Sometimes, either before or after a pellicle is applied, the EAPSM may be cut. After these steps are completed, the completed EAPSM is used to manufacture semiconductors and other products.
Semiconductor manufacturers typically use EAPSMs to transfer micro-scale images defining a semiconductor circuit onto a silicon or gallium arsenide substrate or wafer. The process of transferring an image from a EAPSM to a silicon substrate or wafer is commonly referred to as “lithography” or “microlithography”. Typically, as shown in FIG. 5, the semiconductor manufacturing process comprises the steps of deposition, photolithography, and etching. During deposition, a layer of either electrically insulating or electrically conductive material (like a metal, polysilicon or oxide) is deposited on the surface of a silicon wafer. This material is then coated with a photosensitive resist material. The EAPSM is then used much the same way a photographic negative is used to make a photograph. Photolithography involves projecting the image on the EAPSM onto the wafer. Often, the image on the photomask is projected several times side by side onto the wafer. This process is known as “stepping”, with the EAPSM typically referred to as a “reticle”.
As shown in FIG. 6, to create an image on a semiconductor wafer 20, an EAPSM 10 is interposed between the semiconductor wafer 20, which includes a layer of photosensitive material, and an optical system 22. An energy source, commonly referred to as a wafer stepper 23, is used to transfer an image onto a semiconductor wafer. The energy generated by the wafer stepper 23 is inhibited from passing through the areas of the photomask 10 where the opaque material is present and partially inhibited from passing through the areas of the photomask 10 where the PSM layer is present. By contrast, energy from the wafer stepper 23 passes through the transparent portions of the quartz substrate not covered by the opaque and PSM layers. Current wafer stepper tools are configured to operate at various exposure tool wavelengths (e.g., 248 nm, 193 nm and 157 nm) which are significantly lower than the 365 nm wavelength at which current inspection tools operate.
The optical system 22 projects a scaled image of the pattern of the mask onto the semiconductor wafer 20 and causes a reaction in the photosensitive material on the semiconductor wafer. The solubility of the photosensitive material is changed in areas exposed to the energy. In the case of a positive photolithographic process, the exposed photosensitive material becomes soluble and can be removed. In the case of a negative photolithographic process, the exposed photosensitive material becomes insoluble and unexposed soluble photosensitive material is removed.
After the soluble photosensitive material is removed, the image or pattern formed in the insoluble photosensitive material is transferred to the substrate by a process well known in the art which is commonly referred to as etching. Once the pattern is etched onto the substrate material, the remaining resist is removed resulting in a finished product. A new layer of material and resist may then be deposited on the wafer and the image on the next photomask is projected onto it. Again the wafer is developed and etched. This process is repeated until the circuit is complete.
In the field of semiconductor design, circuit densities on semiconductor wafers have continued to increase while at the same time the minimum feature size on semiconductor wafers have continued to decrease. Manufacturers of optical lithography tools (e.g., wafer steppers) have recognized that the current state of semiconductors design has entered into a sub-wavelength regime and is approaching its resolution limits. In this regard, because optical steppers are now being used for deep, sub-wavelength designs, such manufacturers have developed new technology and equipment to meet these design changes. More specifically, the wafer steppers have been designed in accordance with the transmission properties of current EAPSMs. Inspection tool manufacturers lag behind the wafer stepper manufacturers and have not modified their inspection equipment to meet the optical properties of current EAPSMs. Thus, current phase shift materials meet the optical requirements of exposure tool wavelength (e.g. 193 nm, 157 nm and 247 nm), on the one hand, but do not meet the optical requirements of inspection tool wavelength (e.g., 365 nm), on the other hand. Thus, there has been a long felt need to develop a mask that will meet the optical requirements of both exposure and inspection tools.
Other prior art discloses methods for improving the overall inspection of photomasks, however, this prior art does not address the particular need to make defects in current EAPSMs inspectible in the first instance. For example, U.S. Pat. No. 6,110,623 to O'Grady et al. (“the O'Grady Patent”) addresses the problem where a photomask defect is too small for defect detection by inspection tools. The O'Grady Patent discloses that defect detection during inspection can be improved by depositing a contrast enhancing thin film on the top surface of a finished photomask to alter the finished photomask's reflectivity. In this regard, the top surface of a finished photomask (e.g., one that has already been etched and patterned) is coated with a contrast-enhancing layer to improve the visibility of any defects that exist on the photomask. In other words, the contrast enhancing layer disclosed in the O'Grady Patent is used to make defects appear larger so that they can be more easily seen (assuming that the primary features are inspectible in the first place).
Although useful in improving the inspection of photomasks, this method has some significant drawbacks in its operation and results. In particular, demands for faster production time between the moment a photomask order is placed to the ultimate delivery of the finished photomask make it desirable to reduce the total amount of time spent processing a blank photomask into a finished photomask. Because this enhancement layer is not deposited until after the photomask is processed, the total processing time spent making the photomask is increased. As a result, the overall productivity of a photomask production facility is decreased. Additionally, by depositing the contrast enhancing layer to the photomask after it has been processed, there is an added risk that the photomask will incur additional defects during the deposition of such layer on the photomask. Further, the O'Grady Patent does not address the problem associated with inspection equipment as discussed herein. Specifically, the O'Grady Patent does not disclose the selection of materials for the contrast enhancing layer which will decrease the transmission of light through the PSM layer of an EAPSM to approximately 40-50% or less when compared with transmission through the quartz region of the EAPSM, as required by current inspection tools. Thus, although useful in enhancing the size of defects during inspection, such defects may not be detected in the first instance using the mask of the O'Grady Patent. Accordingly, poor inspection results are still obtained using the photomask of the O'Grady Patent.
Others have attempted to address the problems associated with the prior art by adjusting the material of choice for the PSM layer in a blank photomask. For example, U.S. Pat. No. 5,935,735 and Japanese Pat. Nos. JP 08-304998A and JP 2000-10255 to Toppan (collectively, “the Toppan Patents”) disclose the use of a half-tone zirconium based compound as the choice material for a PSM layer in a “half-tone type phase shift mask”. The Toppan Patents disclose that the zirconium based PSM layer decreases transmission of light at the exposure tool wavelength and inspection tool wavelength. However, the Toppan Patents do not address the problem of obtaining reliable inspection results for EAPSMs (e.g., MoSi based materials) which are more typically used in the semiconductor industry. In this regard, zirconium-based half tone masks are rarely used since as zirconium has been found to exhibit poor etching properties. Thus, the teachings of the Toppan Patents are limited to zirconium-based half tone masks and do not address the problems associated with inspecting EAPSMs more commonly used in the semiconductor industry.
While the prior art is of interest, the known methods and apparatus of the prior art present several limitations which the present invention seeks to overcome.
In particular, it is an object of the present invention to provide an EAPSM having at least one intermediate inspection layer made from materials which improve the inspection results of the mask, while maintaining a sufficiently low transmission at the exposure tool wavelength.
It is another object of the present invention to provide a method and apparatus for improving inspection results of an EAPSM by decreasing the transmission of light at the inspection tool wavelength while maintaining the transmission of light at the exposure tool wavelength.
It is another object of the present invention to solve the shortcomings of the prior art.
Other objects will become apparent from the foregoing description.